Stanford team develops plan for electric highway

With new state regulations demanding an increase in the number of clean and green cars, the future is looking bright for electric cars, such as the Nissan Leaf.

The problem is, most electric cars can't go more than about 100 miles without being recharged.

But a team of Stanford University researchers may have come up with a novel infrastructure design that could solve this problem. They see a future in which magnetic fields could transmit electrical currents on highways, charging cars while they drive.

"Our vision is that you'll be able to drive onto any highway and charge your car," said Shanhui Fan, an electrical engineer at Stanford and a co-author of the paper.

"What makes this concept exciting is that you could potentially drive for an unlimited amount of time without having to recharge," said Richard Sassoon, managing director of the Stanford Global Climate and Energy Project and a co-author of the study. "You could actually have more energy stored in your battery at the end of your trip than you started with."

The idea of sending electrical currents through the air to charge appliances and other devices is not new. Famed 19th-century inventor Nikola Tesla designed a 187-foot tower that could transmit electricity to points miles away. But due to a lack of funding -- and the industrial world's use and then dependence on wiring -- his plans never came through.

This is how it works: Two metal coils, set some distance apart, are tuned to resonate -- or vibrate -- at the same frequency. One of the coils is connected to an electrical source, which generates a magnetic field that makes the other coil start to resonate. This process results in the invisible transfer of electricity through the air from the first coil to the second.

"Wireless power transfer will only occur if the two resonators are in tune," Fan said. "Objects tuned at different frequencies will not be affected."

Members of the Stanford team wondered if they could apply this energy to moving cars -- the trick was to figure out how the coils would need to be placed in a highway and how they should be designed to provide the most effective and directed form of energy.

Two Stanford postdoctoral students figured out that part: A coil bent at 90 degrees and attached to a metal plate can transfer 10 kilowatts of electrical energy to an identical coil 6.5 feet away.

"That's fast enough to maintain a constant speed," Fan said. "To actually charge the car battery would require arrays of coils embedded in the road. This wireless transfer scheme has an efficiency of 97 percent."